The present invention provides solid phosphite compositions with relatively high phosphorus loadings for use as stabilizing additives in polymeric compositions and articles. In particular, compositions in accordance with the invention may comprise tris(4-t-alkylphenyl) phosphites and selected amines, which compositions provide excellent stabilization of polymers, such as polyolefins and the like, particularly in the prevention of color formation during high temperature processing or when exposed to certain gasses during storage, e.g., “gas fading”, and yet which are themselves relatively resistant to hydrolysis.
Organic phosphites are well known and long used in the art as antioxidants for polymeric resins, often in the presence of other antioxidants. One common problem for most phosphites is the tendency to undergo unfavorable hydrolysis upon exposure to moisture or water during storage or handling, which diminishes the activity of the stabilizer, makes it hard to handle and can even cause damage to processing equipment, e.g., corrosion of metal parts, as discussed for example in U.S. Pat. No. 8,048,946, incorporated herein by reference. For example, Kirpichinov et.al, Vysokomolekulyarnye Soedineniya, Seriya B: Kratkie Soobshcheniya (Macromolecular Compounds, Series B: Short Messages), (1970) 12(3), 189-92 states that regarding phosphite stabilizers, “the colour stabilizing properties depend directly on their hydrolytic stability”.
Tris(alkylaryl)phosphite stabilizers having hindered alkyl groups at the ortho positions relative to the C—O—P bond are resistant to hydrolysis due to steric hindrance. One of the most widely used phosphites is tris(2,4-di-t-butylphenyl)phosphite, a white crystalline solid having a relatively high melting point between 180-185° C., and having a bulky alkyl group at both the ortho and para positions. Tris(2,4-di-t-butylphenyl)phosphite has been demonstrated effectively to reduce peroxide induced oxidative degradation for many polymers including polyolefins, polycarbonates, ABS and polyesters etc. The trisalkylaryl phosphite has low volatility that allows for its use at high temperatures commonly required for processing thermoplastic polymers. However, there are processing and compatibility issues with tris(2,4-di-t-butylphenyl)phosphite which compromise its performance in certain polymers and under certain conditions. For example it is known to plate out during processing of some plastics, in particular low melting point plastics, forming deposits on processing machinery surfaces.
Other tris(alkylaryl)phosphites and phosphite compositions, such as the liquid tris(p-nonylphenyl)phosphite, are known which can be used with e.g., polyolefins, without exhibiting the same plate out issues that can be encountered with high melting solid phosphites such as tris(2,4-di-t-butylphenyl)phosphite. Tris(p-nonylphenyl)phosphite (TNPP) however, lacks steric hindrance at the ortho position and is susceptible to hydrolysis.
U.S. Pat. Nos. 7,888,414; 8,008,383; 8,008,384; 8,178,005 and 8,188,170, the disclosures of which are incorporated by reference, disclose liquid phosphites compositions comprising a mixture of alkylphenyl phosphites, typically a mixture of solid phosphites that when appropriately combined exist as a liquid under ambient conditions. These phosphite mixtures are more readily incorporated into many polymer resins than tris(2,4-di-t-butylphenyl)phosphite and are less prone to plate out in, e.g., polyolefins.
Liquid additives however often require processing and storage equipment different from that used with solid additives and can therefore present handling difficulties in some polymer processing facilities. U.S. Pat. Nos. 8,008,385 and 8,309,635, the disclosures of which are incorporated by reference, disclose solid blends of phosphites similar to the previous liquid phosphite blends, but having a different ratio of the individual components. Many of these blends comprise tris(2,4-di-t-butylphenyl)phosphite and/or tris(2,4-di-t-amylphenyl)phosphite in significant quantities.
It is widely believed that while the hindrance of the ortho-substituent in tris(2,4-di-t-butylphenyl)phosphite makes the compound more resistant to hydrolysis, it also sterically encumbers the active phosphorus site making the compound a less kinetically active peroxide decomposer, and thus less effective overall as an anti-oxidant. Further, the presence of a second alkyl group on the phenyl substituent increases the molecular weight of the phosphite considerably by adding essentially inert carbon atoms and thus decreases the weight percent of active phosphorus. For example, tris(2,4-di-t-butylphenyl)phosphite contains 4.8% phosphorus while tris(4-mono-t-butylphenyl)phosphite contains 6.4% phosphorus.
It is known that the addition of certain amines, e.g., tri-isopropanol amine, can improve the resistance of liquid phosphites and certain combinations of solid phosphites to hydrolysis, see for example, the discussion in U.S. Pat. Nos. 8,048,946, 8,008,385 and 8,309,635. However, there remains a commercial need to provide improved antioxidant compositions, in particular compositions which have relatively high phosphorus loadings to provide enhanced antioxidant activity per gram of composition, and which have hydrolytic stability effective to permit reasonable storage of the composition prior to being blended with or otherwise incorporated into polymeric materials, and also maintain sufficient hydrolytic stability once blended with or otherwise incorporated into polymeric materials in order to provide continuing antioxidant activity during the commercial life of the polymeric material in question.